Two classes of contact lenses can be distinguished by the way they are fitted to the eye. In hard lenses the fit is flatter than the cornea itself and the lens rocks back and forth with each eye lid blink, pumping tear fluid and thereby oxygen, as well as cell debris under and from under the lens. Hard lenses are preferred whenever excellent visual acuity is desired and difficult vision corrections are required, for instance in the case of astigmatism. They are however less comfortable for the wearer than are soft lenses, the second class of contact lenses. Soft contact lenses derive their name from their low modulus and draping quality, which allows them to smoothly cover the cornea surface. They are fitted to match the cornea as closely as possible and they are not much disturbed by the eyelid. Because of their tight adherence to the cornea, they have to possess sufficient oxygen permeability to keep the cornea well supplied with oxygen.
In the most common soft lens material -- 40% water containing poly-(2-hydroxyethyl methacrylate) or poly-HEMA -- water provides for sufficient oxygen flux to allow poly-HEMA lenses to be worn on a daily basis. The oxygen permeability O.sub.2.DK of a poly-HEMA hydrogel with 40% water is 6.5 barrers, and for hydrogels with higher water content, for example poly-(N-vinylpyrrolidone) or poly-(vinyl alcohol) copolymers it is 12.5 at 50% water, 15.5 at 60% water and 25 at 70% water. Such higher water content hydrogels allow therefore the manufacture of soft contact lenses for extended wear, up to several months, subject only to periodic cleaning. Unfortunately, high water content hydrogels are also very fragile, especially if they are cut very thin, as they often are in order to increase oxygen flux.
Another class of soft contact lens materials are polysiloxane rubbers (PDMSi), which can have O.sub.2.DK values up to 500 barrers. Several polysiloxane based soft contact lens materials have been described, among them: conventional PDMSi rubbers produced by a hydrosilation cure; PDMSi-polyurethanes and PDMSi-polyacrylate block copolymers. All these materials suffer from an inherent lack of wettability and therefore require some kind of surface treatment to achieve the wettability required for comfortable and safe in-eye wear.
For non-hydrogel, water free contact lens compositions, siloxane units containing polymers have been used both, for soft contact lenses and hard contact lenses with enhanced oxygen permeability; fluorinated groups, which are another group of oxygenflux enhancing moieties, have only been used in hard contact lens materials although they would be preferable to siloxane groups in non-hydrogel soft lenses as well because of their low lipophilicity and low protein adsorption tendencies.
In hydrogel soft contact lens materials on the other hand, siloxane as well as fluorine has been used to enhance the oxygen permeability and many such silicone or fluorine containing hydrogels have been described in the literature for use as contact lens materials. Such silicone and/or fluorine containing hydrogels can possess oxygen permeabilities 3-8 times that of Si-or F-free hydrogels and at the same time retain some of the good wettability of hydrogels. Polysiloxane hydrogels which are water swollen copolymers of 2-hydroxyethyl methacrylate or N-vinylpyrrolidone (NVP) with di- or tri-methacrylate functional poly(dimethylsiloxane) have been described in U.S. Pat. No. 4,136,250 for use as a drug delivery matrix, but also for contact lenses. Copolymers of tri-siloxy-hydroxy alkyl methacrylate with HEMA and/or NVP are described in U.S. Pat. Nos. 4,139,692 and 4,139,513, and copolymers of C.sub.1 -C.sub.4 -dialkylacrylamides with oligosiloxanyl-silylalkyl methacrylates are described in U.S. Pat. Nos. 4,182,822 and 4,343,927, also for use as oxygen permeable hydrogel contact lenses. U.S. Pat. No. 4,711,943 describes essentially similar Si-containing acrylic hydrogels.
Fluorinated hydrogels for use as soft contact lens materials are also known: U.S. Pat. Nos. 4,433,111 and 4,493,910 describe hydrogels and contact lenses obtained by copolymerization of 20-40 mol % substituted or unsubstituted acrylamide or methacrylamide; 25-55 mol % N-vinylpyrrolidone (NVP); 5-20% mol % hydroxy-alkyl(meth)-acrylate; 1-10 mol % (meth)-acrylic acid, and 1-9 mol % of a perfluoroalkyl-alkylene(meth)-acrylate; the perfluoroalkyl groups act to to reduce protein deposition. U.S. Pat. No. 4,640,965 describes hydrogels and contact lenses obtained by copolymerization of hydroxy-fluoralkylstyrene (5.congruent.60%, by weight), with hydroxyalkyl (meth)-acrylates or N-vinylpyrrolidone (40-95%, by weight); the hydroxy groups is necessary to attain the required compatibility.
U.S. Pat. No. 4,638,040 describes the synthesis of 1,3-bis(trifluoro-acetoxy)propyl-2-methacrylate polymers and their use as hydrogel contact lens materials or as ocular implants after hydrolysis. U.S. Pat. No. 4,650,843 describes hydrogel contact lens materials consisting essentially of copolymers of 50-95% (by weight) of 2-hydroxyethyl-methacrylate and 5-35% (by weight) of fluorinated methacrylates with up to 5 F-atoms.
Copolymers of N,N-dimethylacrylamide (DMA) with perfluoroalkyl acrylates or methacrylates are described in copending patent application Ser. No. 215,101 for use as oxygen permeable hydrogel contact lenses.
In all the cited prior-art is the hydrophilic component based of water-soluble vinyl monomers, like HEMA, NVP or DMA, which are copolymerized with silicone and/or fluorine containing monomers or prepolymers. Although a great variety of Si or F containing hydrogels can be prepared with these hydrophilic monomers, they all possess as hydrophilic component a carbon-carbon backbone polymer; for the specific requirements of long-term in-eye use, contact lenses with poly-(ethylene oxide) (PEO) as hydrophilic component would be preferable since PEO is considered to have better biocompatibility and less lipid and protein adsorption problems. Less protein adsorption generally means less discoloration, better wettability and comfort, and generally a longer use life for a contact lens.
Poly-(ethylene oxide) as part of a silicone containing hydrogel is included also in U.S. Pat. No. 4,136,250, in form of dimethacrylate-(PEO)-(PDMSi)-(PEO) block copolymers, copolymerized with HEMA or NVP; the amount of PEO incorporated into the polymer is limited due to the presence of the other comonomers. Similar poly-(propylene oxide)-PMDSi block copolymers are disclosed in U.S. Pat. No. 4,740,533 although this patent is directed toward essentially water free polymers for contact lenses.
Poly-(ethylene oxide) as part of fluorine containing hydrophilic polymers is also described: U.S. Pat. No. 3,728,151 describes PEO block copolymer with perfluoroalkyl (- R.sub.f) acrylates and -methacrylates, obtained by chain transfer polymerization with PEO-dithiols; by virtue of their synthesis method these polymers are linear, non-crosslinked, low molecular weight polymers of the A-B-A block type; their use is in textile finishing, where they import anti-soiling and soil-releasing, self-washing properties. U.S. Pat. No. 4,046,944 describes block copolyurethane-ureas prepared from PEO-diols and PEO-diamines, bis-perfluoroalkyl substituted butane diols and diisocyanates, also for use in textile finishing as soil-release agents. This polymer too is not crosslinked and therefore not a hydrogel, and of limited molecular weight.
No PEO and F-containing hydrogels are described in the prior art for use in biomaterials and contact lenses. This is probably due to the difficulty in making clear compositions of high molecular weight; since the PEO-hydrophile is a pre-polymeric unit of at least 1,000 MW, the F-containing part of the polymer has to be present in a polymeric form as well; block-copolymers of this type are clear only if the blocks are sufficiently short and association between blocks are in size smaller than the wavelength of light. PEO and F-blocks are especially difficult to combine into a clear copolymer because of the inherent incompatibility of their prepolymeric and monomeric precursors and their very different refraction indices.
It has now unexpectedly been discovered that novel, crosslinked, clear, wettable and highly oxygen permeable poly(ethylene oxide) and fluorine or silicone containing block-copolymers can be prepared, if .alpha.,.beta.-divinyl functionalized PEO-prepolymer is copolymerized with perfluoralkyl acrylates or methacrylates and/or oligosiloxy-silylalkyl acrylates or methacrylates together with a poly-ethylenically unsaturated monomer which acts as a crosslinking monomer for the fluorine or silicone containing phase of the novel polymers. The polymerization is carried out in the presence either of a third comonomer acting as a solvent, or a non-reactive solvent or solvent mixture, capable of dissolving all monomeric components.
After synthesis, the polymers can be transformed by equilibration in water into flexible, clear, wettable and oxygen permeable hydrogels, which are useful in biomedical application, as oxygen permeable films and coatings and especially as deposit resistant, highly oxygen permeable contact lenses.
It has further been discovered, that clear block-copolymers with analogous structures can also be made with poly-propylene oxide and poly-tetramethylene oxide as polyether blocks. These block copolymers are strong, flexible, O.sub.2 -permeable and, despite a low (&lt; 5%) water content, highly wettable and are therefore useful in many of the above-mentioned applications.